The influence of rare earth, strontium and calcium on the thermal diffusivity of Mg-Al alloys

The microstructure and thermal diffusivity of as-cast AM60, AE63, AJ63 and AXJ620 alloys were investigated over the temperature range 20-300°C. The microstructure of as-cast AM60 magnesium alloy consists of dendrites of α-Mg solid solution and divorced eutectic containing "islands" of α-Mg and Mg17Al12 phase. In regions adjacent with divorced eutectic, the supersaturation aluminum solute in the α-Mg matrix is observed. The addition of rare earth, calcium and strontium caused the decrease of the volume fraction of Mg₁₇Al₁₂ compound and aluminum content in α-Mg solid solution. Moreover, the intermetallic compounds such as: Al₁₁RE₃, Al₄Sr, Al₃Mg ₁₃Sr, Al₂Ca are observed in the interdendritic regions. The thermal diffusivity of AM60 magnesium alloy increases with increasing temperature up to 150°C, above this temperature the increase is less marked. The change in the slope of the temperature variation of the thermal diffusivity is caused by a precipitation of Mg₁₇Al₁₂ phase in the supersaturation of α-Mg areas neighboring the divorced eutectic α-Mg+Mg₁₇Al₁₂. The addition of rare earth, calcium and strontium caused the increase of the thermal diffusivity and thermal conductivity due to the decreasing of aluminum content in α-Mg matrix.